THE reason for the existence of sex is one of those biological problems which has long perplexed the scientific world, and to-day its solution seems as remote as it did a century ago. Many remarkable discoveries have been made in regard to the microscopic structure and development of the germ cells. We have learned much of the general biology of sex, and the probable evolution of sex in the organic world. And substantial progress has been made in respect to the old problem of the determination of sex. But to the question, Why came there to be two sexes at all? or, in other words, Why did not organisms continue to reproduce asexually as it is probable they once did? we can only offer answers that, at best, are very hypothetical. The bacteria and the blue-green algæ, so far as careful investigation has yet ascertained, reproduce exclusively by the asexual method, usually by fission or the formation of spores. But among the higher plants and in nearly all animals we find the existence of two sexes of very general occurrence. While the fact that sex is absent in the lowest forms of life indicates that evolu has proceeded, at least a certain distance, without its aid, and suggests the possibility of the evolution of sexless forms of a high degree of organization, yet the general prevalence of sex in all but the most primitive organisms points to the conclusion that sex has played a fundamental rôle in the evolution of the organic world. There are many theories as to the part which sex has played, but the profound disagreement among several of these which have secured the widest following is significant of how little is positively established in regard to this subject. While the cause of the development of sex may remain obscure, it is not difficult to point out some of its consequences, although it would be futile to attempt a very accurate picture of what the organic world would be had sex never been evolved. Even if the processes of variation and selection had gone on to the same extent—which is scarcely probable— the absence of sex would have given a very different direction to evolution from that which was actually followed. Many of the most complex structural arrangements of organisms have especial reference to the union of the germ cells. The color and scent of flowers, and their many and beautiful adaptations to secure cross fertilization would never have appeared if plants were propagated exclusively by the asexual method, and this would doubtless have entailed more or less extensive changes in other parts of the organism. In animals the structural peculiarities